The invention relates to a process for the manufacture of ammonium polyphosphate from gaseous ammonia and aqueous phosphoric acid, preferably in the unpurified state and with a concentration of the order of 28% P.sub.2 O.sub.5, through the use of a multiple-stage falling film reactor whose individual stages are kept by means of suitable heating fluids at temperatures adapted to the desired degree of condensation, the preheated ammonia flowing against the phosphoric acid descending in a film within the fall tubes.
Between the individual stages the vaporous water being released by the concentration and by chemical reaction can be separated by intermediate condensation, while the gaseous ammonia (together with a diluting gas in some cases) is fed to the next stage of the falling film reactor after reheating to the particular reaction temperature. The liquids, generally containing ammonium fluoride, which finally emerge from the reactor, are precipitated in an injection condenser.
Ammonium polyphosphate has been gaining in importance as a fertilizer (in solid or dissolved form) and as an additive to detergents, or for use as a flameproofing agent and for other purposes.
The manufacture of this product starts out either from superphosphoric acid and ammonia, or from highly concentrated phosphoric acid containing at least 50 wt-% P.sub.2 O.sub.5 and ammonia. In other processes, ammonium phosphate is concentrated by evaporation in the presence of urea. It is also possible to burn phosphorus in the presence of ammonia or to melt ammonium phosphate together with P.sub.2 O.sub.5.
Dilute phosphoric acid solutions or dilute ammonium phosphate solutions are generally first concentrated by more or less tedious processes, usually multi-step processes, before they are delivered to the apparatus in which the actual manufacture of the polyphosphate is to take place. On account of the great amount of corrosive effects which are usually produced during these processes, they are performed at the lowest possible temperatures and therefore usually at reduced pressure.